"The rumor says that Totoro is the God of Death, so the persons that can see Totoro are actually close to death, or already dead. What that means for the story is that when Mei goes missing and a sandal is found in the pond, Mei actually drowned. When Satsuki is asked about the sandal she cannot face the truth and lies about it not being Mei's sandal. So Satsuki goes on a desperate search for Totoro, calling for him and actually opens up the door to the realm of the dead herself."

This actually makes so much sense, I can't believe it never occurred to me before.